An installation of the type mentioned above is known (EP-A-0 659 690) which comprises a flocculation zone, a zone for mixing the flocculated raw water, in an upward current, with pressurized water delivered by a pressurization-pressure release system, and a flotation zone, in the upper part of which the suspended matter contained in the raw water and brought to the surface by the microbubbles are discharged, this flotation zone being equipped, in its lower part, with a perforated uptake device (floor with or without seal assemblies, collectors, etc.) such that the entire surface of the flotation zone exhibits a uniform and identical flow stream for the clarified liquid.
One characteristic of this type of flotation device lies in the formation of a thick bed of microbubbles by virtue of which the flocculation takes place in two stages, first of all in the flocculation zone and then within the bed of microbubbles by virtue of the large contact mass due to the microbubbles providing, moreover, the separation by flotation of the suspended matter.
One of the limitations to the use of such installations lies in the determining of the dimensions thereof. According to H. J. Kiuri, in an article entitled “Development of dissolved air flotation technology from the first generation to the newest (third) one (DAF in turbulent flow conditions)” published in “Water Science and Technology”, Vol. 43, No. 8, pp 1–7, IWA Publishing 2001, a basic rule for determining the dimensions of flotation cells is that the ratio of the depth (or height H) of the flotation zone, located above the uptake system, divided by the horizontal length L between the water inlet and the opposite wall (H/L ratio) should be greater than 1. The respecting of this rule leads to the construction of installations which are very high and very expensive.
Another difficulty which became apparent during the industrial implementation of this type of installation concerns the formation of a bed of microbubbles of great height in the flotation cell (height of the bed of microbubbles often greater than 3 metres).
This height, firstly, must be greater than a minimum value to ensure good finishing of the flocculation, to optimize the attachment between the microbubbles and the flocs and also to allow the phenomena of coalescence or of agglomeration which cause the enlarging of the microbubbles and therefore the increase in their ascending velocity (30 to 60 m/h) and, secondly, it must be limited in order to reduce the depth of the structures (i.e. their height), and therefore their cost, and high supersaturations of gas. Thus, a height of 1.5 m can result in a supersaturation of +15%, while a height of 4 m can lead to a supersaturation of +40%, which constitutes a major drawback when the flotation cell is inserted upstream of a zone for treatment by filtration, for example through a bed of sand or through membranes.
Factors liable to increase the height of the bed of microbubbles are in particular as follows:                a hydraulic feed of poor quality (heterogeneous) which, for example, leads to the rate of pressurization being increased in order to increase the height of the bed of microbubbles so as to make it more stable, and        a long length for the flotation cell, which brings about great heights (H/L>1 according to the prior state of the art).        